So, way way back in the mists of pre-history, Japan had no writing system (which is what makes it pre-history) but they still had their own spoken language. They would have had, at the time, words for things they saw around them – trees, rocks, rivers, people, et cetera. Then Buddhist scholars arrived in Japan, bringing with them the Chinese writing system, kanji. These kanji were assigned to native Japanese words that already existed, so they’d have a way to write them down, but the kanji already came with their own language attached, too – and also words for more esoteric concepts like government, taxation, armies, et cetera.

Which brings us to kun’yomi and on’yomi. Kun’yomi is the native Japanese reading, the reading for the word that already existed in Japanese before kanji were pasted over the top. On’yomi is the Chinese reading that was imported with the kanji.

So, when do you use which reading? This is the bit where Koichi starts muddying the waters. It’s a very rough rule of thumb, but it goes like this:
- When the kanji’s sitting on its own you use the kun’yomi (for example, 森 (もり, forest) or 夏 (なつ, summer) or 南 (みなみ, south) or 人 (ひと, person)).
- When the kanji’s in a compound word with other kanji, you use the on’yomi (for example, 森林学 (しんりんがく, forestry) or 夏至 (げし, summer solstice) or 南東 (なんとう, south-west) or 人間 (にんげん, human)).
- When a kanji has okurigana on the end (tacked-on hiragana to indicate verb and adjective conjugations) you always, always use kun’yomi, no exceptions (for example, 大きい (おお・きい, big) or 好き (す・き, likeable) or 走る (はし・る, to run)).

Now the trick with Japanese is that all rules have exceptions (except for the kanji-with-okurigana – that’s the exception to THIS rule). Some stand-alone characters tend to use on’yomi, and some compound words will use on’yomi. Some compund words even use a mix of both, just to be confusing. I’ve generally found that the more nature-themed a word is, the more likely it is to use kun’yomi – for example, 川口, river-mouth, uses kun’yomi for both kanji (かわぐち). Proper names tend to use kun’yomi (like 青森 (あおもり) or 小林 (こばやし)), though to confuse matters, many kanji have special readings that are only used in names, called nanori.

My advice, don’t fret about going “this is the on’yomi for this kanji, and that’s the kun’yomi”, because you’re only going to forget the readings without something concrete to hang them on. Instead learn “this is how that word is read”, and you’ll learn the readings as a result. After you’ve got a bit of vocab and kanji under your belt, you’ll start to be able to intuit the readings of words you’ve never seen before, and even kanji you’ve never seen before.

And whew, that turned into an essay. =P

So, numbers usually use on’yomi, even when they’re in bunches – for example 36 = 三十六 = さんじゅうろく (all on’yomi). The main exceptions are 七 and (to a lesser extent) 四 – for whatever reason, their readings tend to be a bit interchangeable.

When a number is attached to the generic “things” counter つ, though, they always use kun’yomi. The つ counter only runs up as far as 10, though. (Technically, kun’yomi numbers do exist beyond ten, they just don’t ever get used in modern Japanese, and I can never remember what they are.)

The 人 counter also has exceptions for 1 and 2 – 一人 = ひとり and 二人 = ふたり, but after that they use on’yomi forever: 三人 = さんにん, 四人 = よにん, 五人 = ごにん et cetera.

P.S. Just realised I mistyped something in my post above – 南東 is south-east. =)

The kun’yomi of 一 is “hito”. “Ichi” is the on’yomi. 一つ = “hitotsu”.

When sticking this into a sentence in Japanese, you’d either write it as “[noun] [number] [counter]” or “[number] [counter] の [noun]” – for example, リンゴ一つ or 一つのリンゴ. If there’s a verb involved, you can write it as “[noun] を [number] [counter] [verb]” – リンゴを一つ食べた.

Also, the use of the つ counter is only used when there isn’t a better counter word in common usage – many objects (especially animals and people) have a better counter word. To count dogs, for example, you’d use 匹 (ひき）. One dog = 一匹 (いっぴき, because with certain numbers, readings get slightly tweaked to make them easier to say).

http://www.tofugu.com/guides/count-anything-japanese/

Shannon wrote:

So, Basically, it comes down to the sentence structure. Not the individual word.

… No, it’s the word, sorry. 一つ is essentially one word. 一匹 is another word used for counting one animal, 一人 is another word used for counting one person. Et cetera.

Adding the tsu changes the pronunciation

That’s… close enough. It’s not so much that the つ changes the pronunciation as that it shows which pronunciation is being used.

To think of another example (which hopefully isn’t going to confuse matters): the character 下 has several kun’yomi, so when we encounter it in a sentence, how do we know which one to use? From the okurigana attached:
- If we see it as 下がる, we know that it’s read as さがる
- If we see it as 下る, we know that it’s read as くだる
- If we see it as 下りる, we know that it’s read as おりる

Admittedly, recognising the other two kun’yomi, した and しも, and telling them apart from the on’yomi, is a little bit more tricky, but trust me when I say you’ll get the feel for it over time, once you’ve started learning a bit more vocab and kanji.

and the order it’s placed in in a Japanese sentence.

Not really, sorry. That’s just how it get used – it’s the same for any counter word. More examples:
私の高校は千人の学生います – わたしのこうこうは せんにんのがくせい います – My high school has a thousand students
あそこに犬二匹がいる – あそこに いぬにひきが いる – There are two dogs over there
それを三つください – それを みっつ ください – Three of those, please

Note that we have a few counter words in English, too – “ten head of cattle”, “three pieces of paper”, “seven pairs of socks”, and you’ll notice the sentence structure here is exactly the same as the “[number] [counter] の [noun]” form in Japanese – but Japanese has a counter word for everything.

Goodness. I feel like I’m being a bother.

No you’re not – that’s what I’m here for. =)

Yep. It’s the word that it’s in, basically. That’s why it’s more important to learn vocabulary rather than trying to learn the readings in isolation.